On 2 Jun 2009 at 9:38, Tim Gaines wrote:
> Has anyone ever been able to repair a vacuum advance or
> retard unit.
Nope, but I'm not sure it's worth the trouble. Well, if it is rigged
to produce a retard at idle then it is part of the emissions system,
so that's a reason, more or less. But without the rest of the system
in place the effect is minimal. When my '80 Spitfire's air pump
seized a long time ago I disconnected everything, then ran it through
state inspection with the vacuum retard hooked up and not hooked up.
It was marginally cleaner with the retard, so I left it in place.
But it was not so much dirtier that I would have worried about it if
the diaphram was broken.
If the diaphram provides advance via manifold vacuum, it is for extra
power at part throttle, sort of. More specifically, it advances the
ignition a bit when the intake air charge is less dense due to the
throttle being not open fully. This give you more power at part
throttle, but it isn't really necessary because if you want more
power you can always just open the throttle further. When the
throttle is fully open you have no vacuum advance at all, i.e. the
same as if it was disconected. So what's the point? It gives you
marginally better fuel mileage at part throttle. Does that matter in
a Spitfire? Well, of course, but not so much as for a heavier car
with a bigger engine.
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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